In some gas turbine applications, there are instances of gas turbine plant operation where the gas turbine pressure ratio reaches the operating pressure ratio limit of the compressor, resulting in compressor surge. These instances may arise in applications where low-Btu fuels or any other fuels with large amounts of diluent injection are used, and/or at cold ambient temperature conditions. The compressor pressure ratio is typically larger than the turbine pressure ratio in that the latter is subject to pressure loss in the turbine combustor.
One common solution that has been used to provide compressor pressure ratio protection is the bleeding off of gas turbine compressor discharge air and recirculating the bleed air back to the compressor inlet. This method of gas turbine operation, known as Inlet Bleed Heat (IBH) Control, raises the inlet temperature of the compressor inlet air by mixing the colder ambient air with the bleed portion of the hot compressor discharge air, thereby reducing the air density and the mass flow to the gas turbine. While this approach eliminates compressor surge, it also reduces turbine output both for the simple cycle operation as well as for combined cycle operation. In the latter case, the reduced gas turbine exhaust flow produces less steam in the Heat Recovery Steam Generator (HRSG) and consequently less steam turbine output. IBH also reduces the thermal efficiency of the gas turbine due to the loss of energy in throttling the compressed air.
This invention provides an improved compressor bleed air method for providing compressor pressure ratio protection, which results in improved output and efficiency of a simple or combined cycle gas turbine power plant (as compared to the IBH approach). This invention is mostly, but not specifically, applicable to gas turbines utilizing standard diffusion flame combustors.
Two embodiments are disclosed herein. Each has applicability to both simple and combined cycle systems.
In a first embodiment, the invention includes bleeding off enough gas turbine compressor discharge air to maintain the compressor pressure ratio limit, and mixing it with the gas turbine (GT) exhaust in a simple cycle system, or at an appropriate location in a combined cycle system (e.g., in the HRSG stack where the two streams have minimum temperature difference). This technique does not increase compressor inlet temperature, and thus does not reduce output as in the case of the IBH approach.
In a second embodiment, where the compressor bleed function is activated for a large percentage of the gas turbine operating period, the method is similar to that described above, except that it uses an air expander device to recover the excess energy associated with the difference between the compressor air discharge pressure and GT exhaust stack (or HRSG) pressure. In addition to the power output increases, this method also results in higher power plant efficiency. In this second embodiment, a portion of the compressor discharge bleed air may bypass the expander via a throttling device to combine with the discharge stream from the expander, to thereby enable plant operation during start up, shut down and during the events when the expander is not operating.
The following are additional optional modifications which may be selected (individually or in an appropriate combination, in both simple and combined cycle operations) based on the economic benefits for a given application. The high pressure bleed air from the compressor is further heated, if required, by means of a pre-heater prior to introduction in the air expander to improve the expander output. The source of this heat can be thermal energy recovered either upstream, such as in the example case of a gasifier with high temperature cooler, or downstream such as the exhaust gas heat recovered from the gas turbine exhaust in a waste heat boiler. Alternatively, the source of heat may include combustion of air and fuel separately supplied to the pre-heater.
In its broader aspects, therefore, the invention relates to a simple cycle gas turbine system comprising: a compressor, a turbine component and a load, wherein fuel and compressor discharge air are supplied to a combustor and gaseous products of combustion are introduced into the turbine component and subsequently exhausted to atmosphere; and a compressor discharge bleed air circuit that removes bleed air from the compressor and supplies one portion of the bleed air to the combustor and another portion of the bleed air to an exhaust stack of the turbine component.